[media] Documentation: v4l: Add hw_seek spacing and two TUNER_RDS_CAP flags

Add a couple of words about the spacing field in the HW seek struct,
also a few words about the new RDS tuner capability flags
V4L2_TUNER_CAP_RDS_BLOCK-IO and V4L2_TUNER_CAP_RDS_CONTROLS.

Signed-off-by: Matti J. Aaltonen <matti.j.aaltonen@nokia.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>